the 500 and the 518 are NOT the same part. Nor is the only difference the first and second gear ratio.

The 500 is essentially an A904 with an OD in the tail shaft.

The 518 is essentially an A727 with an OD in the tail shaft.

they retain all the differences between a 727 and a 904 as the 3 speed versions do.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.